1. Field of the Invention
The present invention relates to a method and an apparatus for continuously forming a functional deposited film of a large area having its composition controlled, by decomposing and ionizing the source gas with the plasma reaction which is caused by the use of a new microwave energy supply device capable of exciting a uniform microwave plasma over the large area.
More particularly, this invention relates to a method and an apparatus for continuously forming a uniform functional deposited film over a large area at a high speed with greatly increased utilization efficiency of the source gas, and specifically, for allowing for the mass production of thin film semiconductor devices with large area such as photovoltaic devices at low costs.
2. Related Background Art
In recent years, the demands of electric power are rapidly increasing all over the world, and the contamination of the environment has become a serious problem as the production of electric power has been actively made to meet such demands.
Hence, in the atomic power generation, which is expected to be used as a power generating method substituted for the steam power generation and has already been put to practical use, a situation may occur where serious radioactive contamination, as particularly seen from a Chernobyl atomic power plant accident, which may injure human bodies and damage the natural environment. Therefore, with great fears for the future of atomic power generation, some countries have rejected new construction of atomic power plants.
Also, in the steam power generation, the consumption amount of fossil fuel, typically coal and petroleum, steadily increases in order to meet increasing demand of electric power, so that the discharge amount of carbon dioxide increases, raising the concentration of green house effect gas such as carbon dioxide in the atmosphere, and bringing about the warmer earth phenomenon, with steadily rising annual average temperature on the earth. Therefore, the IEA (International Energy Agency) has proposed that the discharge amount of carbon dioxide should be reduced 20% by the year 2005.
On the other hand in developing countries, due to the increasing population, the increasing demand for electric power is inevitable, and in present situations, the supply of electric power must be examined worldwide, in conjunction with increased electric power consumption rate per man owing to further progress of electronics on the life style in advanced countries.
Under these situations, the solar-cell power generating method using the solar radiation has been noted as a clean power generating method which can cope with the demand for a future increase of electric power demand, without causing any environmental damage. Because any problems such as the radioactive contamination or warmer earth phenomenon as previously described are not evoked, the energy source is maldistributed, owing to the solar radiation falling on almost everywhere on the earth, and a relatively high efficiency of power generation can be obtained without requiring any complex large equipment, whereby various researches and developments have been made for the practical utilization.
With the power generation method using solar cells, it is fundamentally required that a solar cell has a sufficiently high photoelectric transfer efficiency, with an excellent stability of characteristics, and can be fabricated in mass production in order to meet the power demand.
Hence, in order to meet a necessary electric power in a general home, solar cells having an output of about 3 kW for each household are required, but if the photoelectric transfer efficiency of solar cell is about 10%, for example, the area of solar cells to obtain a necessary output is about 30 m.sup.2. And for example, to supply an electric power necessary for 100,000 general homes, solar cells having an area of 3,000,000 m.sup.2 are required.
Thus, a solar cell which is fabricated by using a source gas such as silane which is easily available and decomposed with the glow discharge to deposit a semiconductor thin film such as amorphous silicone on a relatively cheap substrate such as glass or metallic sheet is superior in the mass productivity, and noted for its possibility of lower cost production, as compared with the solar cell fabrication using a monocrystal silicone, in which various proposals have been made for its fabrication method.
In the power generating method using the solar cell, a method is often used in which unit modules are connected in series or parallel to have a unitary structure in order to obtain a desired current or voltage, each module being required to have no disconnection or short-circuit. In addition, it is important that there is no dispersion of output voltage or current between each module. For these reasons, it is required that the uniformity of characteristics for the semiconductor layer itself, which is the greatest factor of determining the characteristics, may be assured at least at the stage of fabricating a unit module. From the viewpoint of making the module design easier and simplifying the module assembling process, the semiconductor deposited film superior in the uniformity of characteristics over large area can provide for the solar cell at increased mass productivity and greatly reduced production cost.
In the solar cell, the semiconductor layer which is its important constituent is formed as a semiconductor junction such as pn junction or pin junction. Such a semiconductor junction is accomplished by laminating semiconductor layers of different conduction types in sequence, doping a semiconductor layer of one conduction type with a dopant of a different conduction type by ion implantation, or thermally diffusing impurities.
In this respect, considering a solar cell using the thin film semiconductor such as amorphous silicone to be noteworthy as previously described, it is known that in the fabrication, a source gas containing an element which is a dopant such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) is mixed into a main source gas such as silane, and decomposed with the glow discharge, so that a semiconductor film with a desired conduction type can be obtained, and the semiconductor junction can be easily accomplished by laminating these semiconductor layers on a desired substrate in sequence. For the fabrication of the solar cell of amorphous silicones, a method has been proposed in which each semiconductor layer is formed in a separate film formation chamber.
In U.S. Pat. No. 4,400,409 specification, a continuous plasma-assisted CVD apparatus using the Roll to Roll method has been disclosed. With this apparatus, a plurality of glow discharge regions are provided, a sufficiently long, flexible substrate having a desired width is placed along a path in which the substrate penetrates sequentially through each glow discharge region, and continuously conveyed in its longitudinal direction while in each glow discharge region, a semiconductor layer of necessary conduction type is deposited, so that an element having the semiconductor junction can be continuously formed. Note that in this specification, gas gate is used to prevent a dopant gas for the formation of semiconductor layer from diffusing or mixing into other glow discharge regions. Specifically, the glow discharge regions are separated from each other by slit-like separation passage, and means for forming the flow of a scavenging gas such as Ar or H.sub.2 is adopted in the separation passage. By virtue of these respects, it is mentioned that the roll to roll method is suitable for the mass production of semiconductor devices.
However, as the formation of each semiconductor layer is performed with the plasma-assisted CVD method using a RF (radio frequency), there is intrinsically a limit in maintaining the characteristics of film continuously formed and improving the deposition rate of film. That is, even if forming a semiconductor layer having at most a thickness of 5000 .ANG., for example, in depositing on a significantly long substrate, it is necessary to produce a predetermined plasma over large area at all times and to maintain the plasma evenly. By the way, in doing so, considerable skills are needed, and it is difficult to generalize various plasma control parameters associated therewith. The decomposition efficiency and utilization efficiency of used source gas for the film formation are not so high, and act as one factor raising the production cost.
Besides, Japanese Patent Application No. 61-288074 has disclosed a deposited film forming apparatus using an improved roll to roll method. This apparatus is characterized in that a flexible sheet like substrate installed within a reaction vessel is partially formed with a hood-like slack portion, into which active species produced in an activation space different from the reaction vessel and other source gases as necessary are introduced, causing chemical interaction with the thermal energy, and forming a deposited film on an inner face of the sheet-like substrate forming the hood-like slack portion. In this way, by depositing on the inner face of hood-like slack portion, a compact apparatus can be provided. Further, as preactivated active species are used, the film deposition rate can be more increased as compared with a conventional deposited film forming apparatus.
However, this apparatus utilizes a deposited film forming reaction with the chemical interaction under the existence of thermal energy, and to further improve the film deposition rate, it is necessary to increase the amount of introduction active species and the amount of supplying the thermal energy, but there is a limit in a method of supplying a large quantity of thermal energy evenly, or a method of producing a quantity of highly reactive active species and introducing them into a reaction space without loss.
On the other hand, it is a plasma process using the microwave that is recently noted. The microwave makes it possible to further enhance the energy density as compared with the conventional RF, owing to its short frequency band, and is appropriate for efficiently producing and connecting the plasma.
For example, in U.S. Pat. No. 4,517,223 specification and U.S. Pat. No. 4,504,518 specification, there is disclosed a method in which a thin film is deposited on a substrate of small area in the microwave glow discharge plasma under low pressure, in which because of the process under low pressure, it is possible to obtain a high quality deposited film by preventing the polymerization of active species causing the degradation of film characteristics, as well as suppressing the occurrence of powder such as polysilane in the plasma, and attain a great improvement of deposition rate, but there was no specific disclosure for the formation of uniform deposited film over large area.
On the other hand, in U.S. Pat. No. 4,729,341 specification, there is disclosed a low pressure microwave plasma-assisted CVD method and apparatus for depositing a photoconductive semiconductor thin film on a cylindrical substrate of large area with a high power process using a pair of emission-type waveguide applicators, but the substrate of large area is only limited to the cylindrical substrate, i.e., a light acceptor drum for the electrophotography, and not applicable to the substrate having large area and long length.
The plasma using the microwave is likely to cause the nonuniformity of energy because of short wavelength of the microwave, so that various problems for larger area must be solved.
For example, as effective means for attaining the uniformity of microwave energy, there is a slow wave circuit, but it has a specific problem of causing an abrupt decrease of microwave connection to the plasma with increasing distance of the microwave applicator toward the transverse direction. Thus, as means for resolving this problem, a method is attempted in which the energy density in the neighborhood of the substrate surface is made uniform while changing the distance between an processed object and the slow wave circuit. For example, in U.S. Pat. No. 3,814,983 specification and U.S. Pat. No. 4,512,717 specification, such a method has been disclosed. In the former, the necessity of inclining the slow wave circuit at a certain angle with respect to the substrate is described, but the conduction efficiency of the microwave to the plasma is not satisfied. And in the latter, there is disclosed that two slow wave circuits are provided unparallel within a plane parallel to the substrate. That is, they disclose that planes vertical to the center of the microwave applicator are desirably disposed to intersect with each other within the plane parallel to processed substrate and on the line perpendicular to a moving direction of the substrate, and to avoid the interference between two applicators, the applicators should be disposed to be shifted aside in the moving direction of the substrate by half the length of a crossbar of the waveguide.
Also, several proposals have been made to retain the uniformity of the plasma (i.e., the uniformity of energy). Those proposals can be found in the reports as described in Journal of Vacuum Science Technology B-4 (January to February in 1986) pp. 295-298 and B-4 (January to February in 1986) pp. 126-130. According to these reports, a microwave reactor called as a microwave plasma disk source (MPDS) is proposed. That is, they describe that the plasma is shaped as a disk or tablet, its diameter being a function of the microwave frequency. And those reports disclose the following contents. First, there is described the advantage that the plasma disk source can be changed depending on the microwave frequency. However, in the microwave plasma disk source designed to operate at 2.45 GHz, it is impossible to attain a larger area because the confinement diameter of plasma is at most about 10 cm, and the plasma volume is no more than about 118 cm.sup.3. Also, the reports point out that with a system designed to operate at a low frequency of 915 MHz, the plasma diameter of about 40 cm and the plasma volume of 2000 cm.sup.3 can be given by reducing the frequency. Further, the reports point out that the discharge can be extended up to a diameter beyond 1 m by operating at a lower frequency, for example, 400 MHz. However, the apparatus which can attain this condition becomes quite expensive and specific.
Thus, it is possible to accomplish the larger area of plasma by reducing the microwave frequency, but the microwave power source having a high output in such a frequency range has not been generalized, and is difficult to obtain, and quite expensive even if obtained. And the microwave power source of high output with variable frequency is further difficult to obtain.
One of microwave feeding means with an antenna method which is relatively easy to deal with is a microwave plasma-assisted CVD apparatus as described in Japanese Patent Laid-Open Publication No. 57-53857 and Japanese Patent Application No. 61-283116 shown in FIGS. 35 and 36, for example. In either case, the circumference of the antenna is surrounded by a barrel body made of a microwave transparent material, which retains the air-tightness of a film formation chamber to prevent the film deposition on the antenna by introducing the microwave from outside the film formatin chamber, resulting in an increased life of the antenna, whereby a high density plasma can be generated over a wide range of pressure. However, in an apparatus as shown in FIG. 36, there are disposed a substrate 1703 laid on a substrate holder 1702 and microwave power supply means within a reaction vessel 1701. 1704 is a coaxial line as the microwave power supply means, in which the microwave power is introduced through a clearance 1705 provided by cutting away a part of an external conductor of the coaxial line 1704 via a microwave transparent barrel body 1706 into the reaction vessel 1701. However, in this apparatus, it is clearly difficult to form an A-Si film uniformly on the substrate of large area, and there is no specific disclosure. On the other hand, in an apparatus as shown in FIG. 16, within the reaction vessel 1601 are disposed a rod antenna 1602 which is microwave power supply means, a gas supply port 1603, a gas exhaust port connected to a vacuum pump 1604, and a substrate 1607 laid on a quartz barrel body 1606. The microwave power produced by a microwave oscillator is transmitted through a waveguide 1608, and introduced via the rod antenna 1602 and the microwave transparent member 1609 into a space surrounded by the quartz barrel body 1606, exciting the plasma to perform the plasma processing. However, due to the property intrinsic to the antenna that the microwave power will propagate on the rod antenna to be radiated to the space successively, the microwave power is attenuated in a longitudinal direction of the rod antenna, and it is difficult to produce the plasma uniform in the longitudinal direction.
Likewise, as means for exciting a high density plasma efficiently using the microwave, there has been proposed a method in Japanese Patent Application No. 55-141729 and No. 57-133636 in which an electromagnet is disposed around a cavity resonator so that the ECR (Electron Cyclotron Resonance) conditions may stand, and in the institutes, there are a number of reports that a variety of semiconductor thin films can be formed using this high density plasma, and this kind of microwave ECR plasma-assisted CVD apparatus has been put on the market.
In this method of using the ECR, however, it is technically difficult to form uniform deposited film on the substrate of large area owing to the nonuniformity of plasma caused by the wavelength of microwave and further the nonuniformity of magnetic field distribution, because a magneto is used to control the plasma. When the apparatus is tried to be made larger to attain the larger area, the used magnet naturally becomes larger, in which there are various problems to be solved for the practical use, such as the larger weight and space, measures against heat generation, and the necessity of a direct current stabilized power source against larger current.
Further, since the characteristics of the deposited film are not formed at an equivalent level as compared with that formed with the conventional RF plasma-assisted CVD method, and the deposited film formed in the space where the ECR conditions stand and the deposited film formed in the so-called divergence magnetic field space which is out of the ECR conditions are quite different in the characteristics and the deposition rate, it is not said to be a suitable method for fabricating semiconductor devices which particularly require the high quality and the uniformity.
In the U.S. Pat. Nos. 4,517,223 and 4,729,341 as previously described, there is disclosed the necessity of maintaining a very low pressure to obtain the high density plasma. That is, in order to expedite the deposition rate or raise the utilization efficiency of gas, the process under lower pressure is requisite. However, in order to maintain the relation between high deposition rate, high gas utilization efficiency, high power density and low pressure, neither the slow wave circuit as disclosed in the previously-mentioned patent nor the electron cyclotron resonance method is said to be sufficient.
Accordingly, it is necessary to provide a new microwave applicator as early as possible which has overcome various problems associated with the above-mentioned microwave means.
By the way, this method of thin film semiconductor is suitably used, in addition to the use of the solar cell as previously described, for the fabrication of thin film semiconductor devices which are required to have large area and long length such as thin film transistors (TFT) for driving the pixel of liquid crystal display, photoelectric transfer devices for contact-type image sensor and switching elements, and has been partially put to practical use as the key component for image input/output apparatus, and the thin film semiconductor is expected to further spread among the public by the provision of a new deposited film forming method which allows the high quality, the uniformity, the high deposition rate and the large area.